An approach by watercraft provides the best access for a summit bid on Rainbow Peak. Glacier National Park regulations allow non-motorized and low-horsepower watercraft to put into Bowman Lake at the foot Bowman Lake where there is a car campground and boat launch. Once in the water make your way to the right (Southeast) and follow the shoreline until you reach a small bit of land that juts out where a stream empties into the lake. This piece of land is distinctly marked on topographic maps. To avoid confusing another stream with this one note where you are in relation to the peak itself for the stream you're looking for is located in a drainage sided by Rainbow. Game trails are reportedly follow-able on both sides of the creek bed. Shared route beta from various parties tells of the existence of a prominent game trail on the climber's-right side of the creek whereas a fellow Summit Post member said, "...went up and down on the left side on some decent game trails," in route notes provided via a private message. As your party makes it's way up the creek a fork will be encountered. The author suggests continuing straight ahead as opposed to veering left. This will lead you out of the vegetation and to the foot of the peak's East-Southeast scree-covered face. Climb the boulder-strewn creek bed and adjacent terrain eventually gaining the talus and scree-covered upper, treeless section of the peak. From here begin the vertical slog up the scree finding your way through class 3 and 4 terrain. A member of Summit Post is quoted in reference to his parties route as follows, "...I seem to remember a distinct "X" type crack system midway up the SW face above some small snow patches that we climbed up to and followed bottom left to top right. This part of our route could be seen from the lake as we headed back to the foot of Bowman lake." Upon reaching the top of the scree slopes the summit is obvious. There may or may not be a summit register located amongst the loose rock but snow prevented the author from determining this one way or the other. If conditions are right another summit can be bagged from Rainbow. Mount Carter is a short ridge walk to the North-Northeast. The relative remoteness of these mountains makes an enchainment worthwhile for those interested in collecting summits in the Northfork ecosystem. Descent from the summit to the lake is completed by retracing the path taken on the ascent. Take care not to get into some of the steep and tall cliff bands - the location of which should be noted on your approach and ascent. | 
